Heaters such as heating pads are widely available in the market currently. Usually, the heating of a heating line will be automatically interrupted on condition that the temperature reaches the temperature preset by users. Thereby, the temperature of the heaters can be kept within a preset range in order to provide functions such as hot compression while ensure the safety of the users.
In order to control temperature effectively, as described in a (U.S. Pat. No. 5,861,610), an element of positive temperature coefficient (abbreviated as PTC hereinafter) is used as a detection line for sensing the temperature change, and consequently the element can be used together with a heating line for temperature-controlled heating. The temperature of the detection line is compared by a comparison circuit in a controller when the temperature of the heating line is rising or the resistance of the detection line is changed by the high temperature. The electric current input into the heating line can be adjusted based on the comparison result in order to keep the heating temperature within the range preset by users.
Above technique is also disclosed in other (U.S. Pat. No. 6,300,597, U.S. Pat. No. 6,310,322, and U.S. Pat. No. 6,768,086). In one (U.S. Pat. No. 7,180,037), the application of another PCT element or an element of negative temperature coefficient (abbreviated as NTC hereinafter) is disclosed. In this patent, the technical feature distinguished from above conventional techniques is that: a first zero cross signal in response to the detection of a zero crossing of the AC power signal; a second zero cross signal in response to the zero crossing of the phase shifted AC power signal produced by detecting that the resistor of a PTC or NTC element is changed due to temperature; a time difference determinator circuit used to measure the phase-shift time of the first zero cross signal and the second zero cross signal continually; a controller used for calculation and outputting a control signal to render the circuit in conducting or disconnecting state; consequently, the temperature of the heating can be kept within a certain range.
The whole circuit configuration disclosed in above (U.S. Pat. No. 7,180,037) is quiet complicated. Moreover, the temperature-control function can be performed only by the detection and calculation of the time difference determinator circuit and the controller. However, the manufacturing cost would be increased.
In order to overcome above shortcomings, inventor had the motive to study and develop the present invention to provide a temperature-control circuit of a heating line and a temperature-control method thereof, which are distinguished from above prior arts and have advantages of effectively controlling temperature, simplifying component elements, and saving the manufacturing cost.